In high-voltage power transmission systems there is a frequent use of current valves when converting AC power to DC power or vice versa. These current valves are furthermore often controlled in a closed loop.
Power transmission systems do have to be reliable. It is important to limit possible faults as much as possible. Because of this the control of the current valves in these systems, which is performed through a converter control device, does in many cases have to be redundant. In known redundant systems, there are provided two parallel converter control devices, where one can take over control when there is a fault in the other. These converter control devices are then arranged in an Active/Stand By configuration using a separate switch-over logic circuit.
An example of this type of solution is described by Bertoni et al in “MACH-2 Modular Advanced Control 2nd Edition, Transmission and Distribution Conference and Exposition: Latin America, 2004 IEEE/PES. 8-11 Nov. 2004, page 884-889.
The normal way to provide such switch-over is thus through providing a separate switch over logic circuit, with which the two converter control units communicate, using for instance hand-shaking. Such a switch-over logic circuit should furthermore allow the continuous operation of the current valves in the power transmission system also during the switch-over. This is not so easy to do when this switch-over logic circuit is in the process of deciding to switch over during closed-loop control.
Moreover, if also this switch-over logic circuit becomes faulty, the system will have to be shut down if the switch-over logic circuit is to be repaired or replaced, which is highly undesirable.
Therefore, there is a need for providing redundant control of at least one current valve in a power transmission system, where the use of a separate switch-over logic circuit can be avoided.
In view of what has been described above there is therefore a need for providing a different way to determine the change in insulating ability, which considers also the cause of the changes.